Expression Variables not recognising updates

I am attempting to setup an Expression Variable @Current_Session_ID that is used to fill a field from a form. This works initially, however if while I have the project open, change the Expression Variable Value, then return and add another feature it adds the old value from the variable, not the updated value. If i do this to a layer i have not been working on, it will use the correct new value. Why is this the case? Is there a way around this?

The application here is for the @Current_Session_ID is to allow users to give it a value at the begining of a survey session. This means all the items recorded while the variable is set could automatically be given that value. Is there any other alternatives to this, not using the expression Variables?